The book is composed of an introduction, several chapters on the nature of holiness, followed by expository sermons that illustrate these truths.

True holiness is not achieved by works, but is rooted in union with Jesus Christ. Faith in Christ is the root; holiness is the fruit.

While emphasizing that only Christ's righteousness justifies us, Ryle maintains that holiness is the "only sure mark of God’s election" and is "absolutely necessary" as evidence of true faith.

Ryle defines holiness not as sinless perfection, but as the inner spiritual work of the Holy Spirit in a believer's heart, resulting in practical obedience and likeness to Christ.